1. Field of the Invention
The present invention relates to a lithographic apparatus, an excimer laser and a device manufacturing method. This invention also relates to a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of radiation. The projection system may, for example, be a refractive optical system, or a reflective optical system, or a catadioptric optical system, respectively including refractive optical elements, reflective optical elements, and both refractive and reflective optical elements.
Generally, the projection system comprises a device to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system.
An illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The illumination system of the apparatus typically comprises adjustable optical elements for setting an outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of an intensity distribution upstream of the mask, in a pupil of the illumination system. A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate.
Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is referred to as the critical dimension (“CD”).
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for both semi-dense and isolated features may limit the process latitude (i.e., the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask having the same nominal critical dimensions will print differently depending on the amount of defocus (out of a plane of best focus) of the part of the target portion where the feature is imaged, due to, for example, substrate topography, image curvature or substrate unflatness.
A difference in printed CD between two similar features such as contact holes at two respective, different locations on a substrate having corresponding different focal positions will be referred to hereinafter as a CD-focus error. For example, a contact hole having a particular contact hole size and disposed at a first position in the pattern, will print differently from the same feature having the same size and disposed at a second position, when at respective conjugate first and second positions at substrate level the respective exposed substrate areas are disposed at respective different focus positions. Hence, when both contact holes are to be printed simultaneously, a position dependent variation of printed CD is observed. Data describing a specific CD-focus dependency are generally represented by a plot of CD versus focus for a constant exposure dose, and referred to as a Bossung curve. The phenomenon CD-focus error is a particular problem in photolithographic techniques.
Conventional lithographic apparatus do not directly address the problem of CD-focus error. Conventionally, it is the responsibility of the users of conventional lithographic apparatus to attempt to maintain CD-focus error within tolerance by compensating for a defocus (for example by adjusting or varying the focus position of the substrate during an exposure), or by optimizing the setting of apparatus optical parameters, such as the NA of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that focus dependency of dimensions of printed isolated and semi-dense features is reduced. However, such a measure to reduce CD-focus error may lead to an increase of other lithographic prosess errors or sensitivities, and may therefore still adversely affect the process latitude.
Given a pattern to be provided by a patterning device, and to be printed using a specific lithographic projection apparatus including a specific radiation source, one can identify data relating to CD-focus error which are characteristic for that process, when executed on that lithographic system. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is a problem of mutually matching the corresponding different CD-focus dependencies, such as to reduce CD variations occurring in the manufacturing process.
An actual CD-focus dependency as described above may be varying in time. For example, due to lens heating the aberration of the projection system may vary, and or due to heating and other instabilities properties such as illumination settings, and exposure dose of radiation energy may vary in time. Therefore there is the problem of controlling and keeping within tolerance a desired CD-focus dependency.
A lithographic process is generally characterized by a process latitude or process window such as, for example, an Exposure-Defocus window, also referred to as an ED-window. An ED-window indicates the available depth of focus in combination with the allowed amount of residual error in the exposure dose of irradiated target portions for a given tolerance on CD.
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD due to focus variations and exposure dose variations may limit the process latitude. In general the useable focus-range of an ED-window is asymmetric around a focus position referred to as the Best Focus or BF. At best focus, a change of CD as a function of a change of focus position of the substrate is smallest or even zero (in the latter case the Bossung curve is locally “iso-focal”, i.e., parallel to the focus axis of the Bossung plot). Typically the available range of focus for contact holes tends to be located asymmetrically around the position BF of best focus, and the CD-focus dependency is then asymmetric with respect to best focus. This corresponds to a process whereby printed contact holes close earlier in one defocus direction, as compared to the other defocus direction. A corresponding Bossung curve is typically shaped as a tilted and curved line segment. An asymmetric CD-focus dependency is limiting the lithographic process latitude, in particular for processes applied to wafers having a considerable topography.